Cut Halo 3 levels
From Halopedia, the Halo wiki
During the development of Halo 3, a number of levels were cut or changed dramatically.
Cut campaign levels
Guardian Forest was a cut level set either between The Ark and The Covenant or after The Covenant but before Cortana. The level was set in a large forest on Installation 00 and was to feature a new enemy, the Guardian.
Early campaign levels
An early version of Cortana was supposed to feature more High Charity specific geometry but this was cut. Instead parts of the level Floodgate was split a part with some of that geometry becoming the Cortana that was released.
A number of multiplayer maps were considered for inclusion in Halo 3, including several that did not make the final cut. Notably, the maps "Wall of Cotton" and "Shaft" do not match any known development names for released multiplayer maps, nor fit the theme of any released in the final game. The document also shows remakes of the maps Hang 'Em High, Blood Gulch and Lockout as considered for inclusion - the latter of which eventually making its way into the game as Blackout.
In the Halo 3 component of Halo: The Master Chief Collection, a number of seemingly-leftover
Little is known of "Gauntlet" - the map was used in development to test a variety of objective-based gamemodes that ultimately never appeared in the final release of Halo 3. Interestingly, concept sketches for the map layout suggest the idea of Halo 3 including several classes including "Assault", "Rifleman" and "Warthog Dude".
The map internally named "Monkey Mountain" was built for the Capture the Flag gametype, and consisted of the attacking team assaulting up a hill (similarly to High Ground) to capture the flag. However, the map had a unique twist in that the attackers could then drop the flag off a cliff next to the base into a river below, at which point a boat could drive the flag back home, similarly to the design of Halo 2's map Relic. The map was ultimately scrapped due to the river mechanic not working well enough to justify including the map. Monkey Mountain was also to include a rideable elevator of some kind.
The map internally codenamed "Warthog Inc" was focused on the Territories gametype, and one of the last maps cut from Halo 3. The map was to be set in a location where the titular Warthog was built and tested, and was reported by Chris Carney as "playing really well"; consisting of a collection of five 1-2 storey buildings each containing a territory zone. The map also contained a Warthog "test track" which allowed the player to use vehicles in a medium-sized map, but made movement predictable as to keep them balanced versus infantry. The map was ultimately scrapped as it never hit an "amazing" level of gameplay.
Interestingly, Warthog Inc was also included (albeit unplayable) in the files of the Halo 3 beta. Additionally, it seems that Warthog Inc's early blockout geometry can be seen being playtested by Bungie staff in the ViDoc Is Quisnam Protero Damno!. In one shot, the level is seen selected in the custom game lobby, and the geometry shown matches the level icon found in the Halo 3 Delta build. Design documentation shared by Marcus Lehto shows Warthog Inc as intended for 4-8+ players, and was in development by the time Halo 3 ended pre-production.
Development and testing levels
The Lost City
The Lost City was a visual look-development level. The level was based around the idea of a Forerunner city, composed of a single building. The space had grown immense in size. An art pass was done on the level in 2005 before it was cut.